Sunday, December 27, 2009

Nuclear weapon tests / Sources of pollution

Nuclear weapon tests

When nuclear weapons were tested above ground, they propelled a variety of radionuclides from hydrogen-3 (tritium) to plutonium-241 into the upper atmosphere. From there, the radionuclides transferred slowly to the lower atmosphere and then to the Earth's surface. Around 500 atmospheric explosions were conducted before the limited test treaty was enacted in 1963, with a few more until 1980. The concen­trations of radionuclides in air, rain and human diet are now much lower than the peak values in the early 1960s.

Globally, the most important radionuclides from testing in terms of human exposure are now carbon-14, strontium-90 and caesium-137. Minute quantities of these are ingested with food and drink. Residual activity from radionuclides in the ground that emit gamma rays also causes a slight degree of human exposure. Internal and external irradiation contribute about equally to the global average effective dose of 0.005 mSv in a year. This compares with a peak of more than 0.1 mSv in 1963. Some groups of people who receive significantly higher, doses from global fallout than average have been identified. For example, it was' found in the 1960s that reindeer and caribou herders in northern Europe and Canada received significantly higher doses than other people, because they eat the meat of animals that eat lichen, which is a very efficient collector of airborne caesium-137. The global collective dose from weapon tests fallout is now about 30 000 man Sv annually, assuming a world population of 6 000 million.

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